A cellular phone and a portable information terminal device have a piezoelectric vibrator using a crystal or the like as a time source, a timing source for a control signal, a reference signal source and the like. Various piezoelectric vibrators of this type have been provided, and among them, a known piezoelectric vibrator includes a piezoelectric vibrating strip of a tuning fork type.
The piezoelectric vibrating strip of the tuning fork type includes a pair of vibrating arm portions extending in parallel and a base portion supporting the base end portions of the pair of vibrating arm portions. An electrode film is formed on the outer face of the piezoelectric vibrating strip. When a voltage is applied to the electrode film, the pair of vibrating arm portions can vibrate at a predetermined resonance frequency in the direction in which they are brought closer to or away from each other.
In recent years, as the device having the piezoelectric vibrator mounted thereon is reduced in size, a reduced size is desired in the piezoelectric vibrating strip.
For example when the width of the vibrating arm portion is reduced, however, the width of the electrode film (exciting electrode) formed on the vibrating arm portion is also reduced to increase the crystal impedance (CI) value, thereby degrading the quality of an output signal.
The resonance frequency F at which the pair of vibrating arm portions vibrate is represented by F=k(W/L2) (where k represents the coefficient, W represents the width of the vibrating arm portion, and L represents the length of the vibrating arm portion).
When the length or the width of the vibrating arm portion is changed, the resonance frequency F is shifted to change the vibration characteristics. The attempt to reduce the size of the piezoelectric vibrating strip needs to be made carefully so as to avoid the shift of the resonance frequency.
To address this, a known approach to reducing the width of the vibrating arm portion is to form a groove portion in each of the upper and lower faces of the vibrating arm portion (see, for example, JP-A-2009-81520).
Since the formation of the groove portion provides the arrangement of a pair of exciting electrodes opposite to each other on the sides of the groove portion, the electric field can efficiently act in the opposite directions. This can increase the electric field efficiency even when the width of the vibrating arm portion is reduced, so that the size reduction can be achieved with the resonance frequency maintained.
As described above, the formation of the groove portion in the vibrating arm portion is effective in increasing the electric field efficiency to reduce the CI value. Particularly, as the size of the groove portion is increased relative to the vibrating arm portion, the CI value can be effectively reduced.
On the other hand, the rigidity of the vibrating arm portion is reduced by forming the groove portion. Particularly, when the width of the groove portion is increased, the rigidity of the vibrating arm portion is reduced accordingly. For example, if an external shock or the like is given to the piezoelectric vibrating strip, the vibrating arm portion may be deformed or broken. When the groove portion is formed near the portion connecting the base end portions of the vibrating arm portions to the base portion, the CI value is effectively reduced but the vibrating arm portion is easily deformed or broken.